Magnetic prospecting is a quick and convenient physical prospecting method and effectively used for various fields including a resource prospecting, geological structure investigation (geological survey) and civil engineering. A borehole magnetic field survey is a survey method for grasping a physical characteristic inside the earth by drilling through a deep hole in the ground, for which a borehole magnetometer is used.
The borehole magnetometer is a magnetic force measurement device used for prospecting underground resources by drilling through a deep hole in the ground or ocean floor, or for measuring changes in past terrestrial magnetic field (terrestrial magnetism) remaining in rock layers or sedimentary layer. The borehole magnetometer includes a magnetic (force) sensor capable of measuring the magnetic force, and a small fluxgate magnetometer is generally used for measuring three directional (X, Y, Z) magnetic forces.
A conventional borehole magnetometer is formed by including a housing, a frame mounted inside the housing, various electronic devices mounted on the frame and sensors. The frame is a processed plastic round rod where an upper surface is formed with grooves for inserting electronic devices and a bottom surface is grooved with grooves for tightly inserting a magnetic force sensor. The frame remains in an unprocessed rod state at a center thereof, which is to separate a magnetic force generated from the upper electronic devices from the bottom magnetic force sensor so that the magnetic force can be less influenced to the bottom magnetic force sensor. The housing is formed by including a pipe, and upper and bottom lids to cover upper and bottom sides. The upper lid of the housing is connected by a cable for supplying a power to the borehole magnetometer and for receiving and transmitting data.
In order to ascertain whether magnetic force data from depth between ascent and descent are mutually corresponding when a magnetic force is measured through the borehole magnetometer, a cable is used where the borehole magnetometer is descended to a borehole to obtain a magnetic data and to obtain the magnetic data by ascending the borehole magnetometer.
The borehole magnetometer is scratched and hit to a borehole wall when reciprocating inside the borehole to allow the housing to be exposed to shocks. However, a magnetic sensor of conventional borehole magnetometer is physically fixed to a frame inside the housing to disadvantageously allow the shock applied to the housing to be directly transmitted to the magnetic sensor through the frame.
Furthermore, another disadvantage is that the shock received in response to physical properties of the borehole wall is not constant and noise generated by the magnetic sensor is not constant either. As a result, a technical development is imperative that a physical shock generated from the borehole wall be minimized on the magnetic sensor inside the borehole magnetometer.